1. Field
Electrically controlled, engine exhaust gas turbochargers.
2. Description of the Related Art
Turbochargers use an engine's own exhaust gases to compress and, thus, increase the volume of air entering the engine to increase engine efficiency.
Engine exhaust gases from the exhaust manifold drive a turbine at high speed. Turbine rotation rotates a shaft, which is shared with a compressor. The compressor compresses outside air and delivers it to the engine's intake manifold. Compression causes more air and thus more oxygen to enter each combustion cylinder. Consequently, the engine operates more efficiently, at higher horsepower and torque and with lower cylinder displacement than conventionally aspirated engines. Thus, lighter engines using less fuel can provide equal performance than engines without a turbocharger provide.
Few diesel engines in new vehicles today operate without a turbocharger. Turbochargers also are becoming more common on gasoline engines. Other, non-vehicle engines also benefit from turbochargers.
At low engine revolutions, the exhaust flow may not drive the turbocharger sufficiently to obtain sufficient compressor rotation to force enough air from the compressor to the engine's intake manifold. Thus, when a driver accelerates quickly from idle or low engine revolutions to high engine revolution, a turbocharger lags until the volume of exhaust gases from the higher engine revolutions reaches the turbocharger. Accordingly, just when an engine is called upon to deliver more power, the lagging turbocharger supplies less then the desired airflow to the engine's combustion cylinders.
Because of these issues, some have proposed that turbochargers operate under electrical control from an electric motor within the turbocharger. See, for example, Kawamura, U.S. Pat. No. 4,769,993 (1988) and Halimi, U.S. Pat. No. 5,605,045 (1997). At idle when the driver wants to accelerate rapidly, an electric motor can accelerate the compressor quickly to supply sufficient air to the intake manifold and the combustion cylinders. After the engine reaches higher rpm and produces higher exhaust volume for adequate turbine rotation, the turbocharger does not rely on the motor. Then, the motor could function as a generator or an alternator and convert the turbocharger's rotary motion into electrical energy to supply at least part of the vehicle's electrical needs. For example, the motor functioning as a generator or an alternator could charge batteries or supply other electrical needs of a hybrid vehicle.
Significant challenges still exist so that the motor continues to function in the harsh, high temperature, high speed environment of a turbocharger. The high gas temperatures on the turbine side of a turbocharger (≈1050° C. in gasoline engines; lower in diesel engines), adversely affects the entire structure. In addition, the compressor side of the turbocharger causes significant temperature increases because the increase in air pressure raises the air temperature, up to an increase of about 180° C. Moreover, resistance heating in the motor's stator adds to the turbocharger's heat load.
Turbocharger manufactures have built standard turbochargers with designs and materials to account for these high temperatures. Nevertheless, those structures and materials may not protect internal motors inside electrically controlled turbochargers adequately.
High temperatures affect the motor in several ways. Insulation on the electromagnets' coil wiring can melt. The melting exposes bare wiring and can short the coils. If the motor shorts, the electrically controlled turbocharger fails to function. Electric resistance of copper wire also increases linearly with increased temperature. This higher resistance at high temperatures decreases motor efficiency and causes the coils to generate more of their own heat.
High-speed rotation also causes problems. Because the turbine and compressor are adjacent each other in conventional (non-electrically controlled) turbochargers, the shaft connecting the turbine and compressor is relatively short. With longer shafts accommodating an electric motor, slight imperfections in the shaft become magnified at the high rotational speed at which turbochargers can operate. Centrifugal force follows the following equation:F=m·ω2r  (1)where m is the mass, Ω is the rotational speed (in radians per unit time) and r is the radius. For a rotating shaft assembly mass, the equation becomes more complex. Nevertheless, the equation still shows that as the shaft's radius increases, which increases the mass, the centrifugal force also increases. In addition, as the shaft length increase, the shaft becomes more flexible and its natural frequencies drop. Thus, resonance may occur at lower speeds.
Unless the shaft is perfectly round and uniform, resulting unbalances cause centrifugal force, which tends to vibrate the shaft. Any oil residing on the shaft—oil within the motor housing is discussed later in this application—also may lead to slight imbalances of the shaft. As the shaft passes through natural frequencies, unbalances can amplify resonances that can affect the turbocharger adversely.
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